1. Field of the Invention
The invention is in the field of integrated optics. More in particular, it relates to a method for the fabrication of a waveguide structure with the application of double masking, which waveguide structure includes a symmetrical part and an asymmetrical part, and to a pair of mask patterns for applying the method.
2. Prior Art
In an optical input section of a coherent optical receiver a mixing element is present for mixing a locally generated optical laser signal and an incoming optical communication signal. It is known from reference [1] that in an integrated version of an optical input section of this type such a mixing element can be constructed as an asymmetrical X junction which in fact is composed from a symmetrical and an asymmetrical Y junction, both having single-mode wave-guiding branches and coupled via a dual-mode connecting channel. An asymmetrical X junction of this type is polarization- and wavelength-independent, has a high degree of fabrication tolerance, and operation is independent of the length of the dual-mode waveguide channel. The function of the asymmetrical Y junction is that of mode filter or splitter. For the purpose of operating in this way, the single-mode branches of the asymmetrical Y junction have different propagation constants, for example due to a difference in width. From an optical signal which, in the dual-mode waveguide channel propagates in the direction of the asymmetrical Y junction in accordance with the fundamental guided mode, will continue to propagate in that branch of the asymmetrical Y junction, which has the highest propagation constant, while such an optical signal which, in the dual-mode waveguide channel, propagates according to a first-order guided mode, will continue to propagate in the other branch. It is a known problem, however, that if the vertex of such an asymmetrical Y junction is not sufficiently sharply defined, particularly as a result of unduly limited photolithographic resolution in the vertex during fabrication, the filtering or splitting action is insufficient. In reference [2] and in a patent application by applicant, unpublished at the priority date of the present invention, see reference [3], solutions for this problem are described on the basis of a technique of double masking. According to these solutions, an asymmetrical Y junction having a sharp vertex is obtained by defining the two branches of the Y junction in different mask layers which overlap one another. If applied to the implementation of an asymmetrical X junction, a first mask pattern could define the symmetrical Y junction, the dual-mode connecting channel and a branch of the asymmetrical Y junction, and a second mask pattern could define the remaining branch of the asymmetrical Y junction. Reference [3] also states (page 9, lines 1-6), that the two mask patterns do require alignment features, but that these primarily relate to the angle between the two branches of the asymmetrical Y junction, rather than the relative position of the two branches with respect to one another. Such positional alignment tolerance for the orientation of the mask patterns with respect to one another is most convenient during fabrication. Simulations of the asymmetrical Y junction according to a method which is known under the name beam propagation method have shown that in a direction according to the signal propagation direction in the connecting channel said alignment tolerance does indeed exist, but that in a direction perpendicular thereto a mutual displacement of the two mask patterns in the order of a few microns with respect to one another results in large variation in mode suppression (typically from -16 dB to -30 dB for a shift of .+-.1 .mu.m with respect to the most favourable position). For a wavelength- and polarization-independent operation of the asymmetrical X junction as a whole, such a variation even if only in the mode suppression in the asymmetrical Y junction is, however, unacceptable. In order to make alignment tolerant fabrication of the asymmetrical X junction possible, said mode suppression in the asymmetrical Y junction must however be better, over the entire tolerance range, than a preset value (typically -30 dB). In that case, the splitting ratio in the X junction, when used as a mixing element in a coherent receiver, will at all times deviate within acceptable limits (typically 0.5 dB) from a fifty-fifty distribution, which keeps the deterioration of the coherent detection within limits (typically up to 1 dB).
The simulations have further shown that the variation in mode suppression depends not so much on the asymmetrical Y junction itself, but on a symmetry-breaking transition of the dual-mode connecting channel as far as the vertex of the asymmetrical Y junction. The symmetry-breaking enables coupling, in said transition, between guided modes of the zero and first order, as a result of which the mode suppression is affected negatively. The extent of the symmetry-breaking is closely related to the geometrical shape of the transition, which can vary considerably with the relative position of the two mask patterns used during fabrication with respect to one another. Let FIG. 1 illustrate this. It shows, in a top view, a top face 1 of a layer stack to be processed, to which a first mask pattern 2 and a second mask pattern 3, which overlaps the first mask pattern, have been applied, which together define an asymmetrical Y junction. The first mask pattern 2 consists of a first part 2.1 and a second part 2.2, for the definition of the dual-mode trunk and one of the lateral branches of the asymmetrical Y junction, respectively. The second mask pattern 3 defines the other lateral branch of the asymmetrical Y junction. The space of the top face 1 between the second part 2.2 of the first mask pattern 2 and the second mask pattern 3 forms a vertex V.sub.1 having a vertex angle .alpha.. For the top face 1, an (xz) coordinate system has been specified, whose z-axis coincides with an axis of symmetry of the first part 2.1 of the first mask pattern 2. As seen in a direction according to the z-axis, three sections can be distinguished in the coverage of the top face 1, viz. a symmetrical section S.sub.1, a symmetry-breaking transition section S.sub.2 and, from vertex V.sub.1, an asymmetrical section S.sub.3, the asymmetrical Y junction proper. The length and the shape of the symmetry-breaking transition section S.sub.2 are strongly dependent on the position of the second mask pattern with respect to the first mask pattern, i.e. on the degree of accuracy with which, during fabrication, the two mask patterns are aligned with respect to one another. Since for an asymmetrical Y junction which functions as a mode splitter, the vertex angle is small (typically .alpha..apprxeq.5 mrad), the alignment accuracy particularly in a direction perpendicular (x-axis) to the axis of symmetry (z-axis) is much more important than that in a direction according to the axis of symmetry. The formation of such a symmetry-breaking transition section can be counteracted by the two mask patterns being positioned with respect to one another with a very high alignment accuracy, particularly in the x direction. Such an alignment accuracy, in which the symmetry-breaking transition section is not or virtually not formed, cannot be achieved in practice, however. Alternatively, the effect of such symmetry-breaking could be diminished by a further reduction in the vertex angle .alpha., which must go hand in hand, however, with an undesirable lengthening of the asymmetrical Y junction and thus of the asymmetrical X junction in its integrated form.